Such double seat valves are not only used in the food industry, but for example also for cosmetic or medical products, to connect flow paths or reliably separate them from each other, where e.g. in case of a product change or after certain operating times, external and/or internal cleaning cycles must be performed. Here, important requirements on the double seat valve have to be met, among other things in that there does not occur any communication between the flow paths in the closed state of the double seat valve and during cleaning cycles, that any leakage that might occur in the closed state under no circumstances get from the one flow path to the other one, that no uncontrolled loss of the separation of the flow paths can occur in cleaning cycles, and that, in the cleaning cycles using a cleaning medium, all components of the double seat valve that have come into contact, for example, with food, a leakage or the cleaning medium, can be perfectly cleaned, optionally even under aseptic conditions.
At least two different types of double seat valves are common. In the one type, the first valve disk fulfills a seat valve function with an axial or an axial and radial seal in the or at the seat, while the other valve disk provides a sliding valve function with an only radial seal in the seat. In the other type, which is suitable, for example, in aseptic conditions, however, the second valve disk also provides a seat valve function with an axial or an axial and radial seal. In the closed state of the double seat valve, the two valve disks together define a leakage space in the seat which can be connected with the external surroundings, receives possible leakages and thus prevents leakage from getting from one flow path into the other one. During the opening cycle, the second valve disk initially starting with its opening movement takes along the first valve disk, while the leakage space is sealed to the outside by a central seal as soon as both valve disks depart from the seat and the flow paths are connected. The closing cycle of the double seat valve is usually performed by spring assemblies which act at least on two of the three pistons. For the double seat valve type in which the second valve disk provides the sliding valve function, the first piston acts as main piston for the opening cycle, while the second and the third pistons are provided as vent pistons for the cleaning cycles of both valve disks. In the double seat valve type whose second valve disk also provides a seat valve function, the first piston acts as main piston for the opening cycle and optionally as vent piston for a cleaning cycle of a valve disk, while the third piston functions as vent piston for the cleaning cycle of the other valve disk, and the second piston secures the valve disk not subjected to a cleaning cycle in the closed position and limits the stroke during the cleaning cycle of the one valve disk. There are important requirements on the drive device in that short switching times are achieved in particular in the cleaning cycles, that the drive device is constructed in a manner as compact as possible as the space in the surrounding area of such double seat valves is often very restricted, and that exactly defined vent positions of each valve disk during the cleaning cycle and a preferably low pressure medium consumption in the cycles can be achieved. Normally, compressed air is used as pressure medium for controlling the drive device.
In the drive device known from DE 42 43 111 A for an aseptic double seat valve, the second piston is guided to be movable inside the first piston in a sealed manner in the drive device housing, so that the first chamber is defined between the first and the second pistons. The pressure transmission path into the first chamber extends through the space in the drive device housing which contains the main spring assembly. This does not only result in an undesired long switching time during the cleaning cycle as pressure builds up with some delay in the large space, but also in a high consumption of compressed air combined with a vent position of the valve disk which is in an undesired manner defined depending on time.
Drive devices known from WO 2005/093298 A and WO 2005/093299 A can be optionally used for both types of double seat valves mentioned in the beginning by rearranging several components of the drive device located inside. The second piston is movable inside the first piston in a sealed manner and together with the first piston defines the first chamber. In one embodiment, the pressure transmission path extends centrally through the drive device housing and across several flow deflections into the first chamber. In another embodiment, the pressure transmission path extends laterally into the drive device into the first chamber, though not directly into the first chamber but across several deflections and through the first piston.
Drive devices known from DE 10 237 236 A and EP 1 525 415 A each for only one double seat valve type comprise two drive device housings placed one after the other in the axial direction of the complete assembly of drive device and double seat valve, the housings being connected to each other in a sealed manner, one of the housings containing the main spring assembly and the first piston, and the other one containing another spring assembly and the second and third pistons and three pressure transmission paths. The pressure transmission path to the first chamber extends centrally axially through the one drive device housing pointing away from the double seat valve. The drive device is structurally complicated and occupies an unsuitably large overall size in the axial direction.